In the design of many electromagnetic clutch assemblies, there is often included an armature member which is axially movable into and out of engagement with an inner body, which engagement establishes drive through the clutch. The axial position of the armature is controlled by an electromagnetic coil which draws the armature into engagement with the inner body. This axial movement is generally against the disengaging bias of a biasing spring. The armature, at the same time, transmits torque through the unit and, thus, must be rotatably connected to either the driven or driving hub members. These requirements for the mounting of the armature, i.e., that the armature be spring biased out of engagement with the inner body and, at the same time, be rotatably connected to a driving or driven hub while being mounted for relative axial movement, is often provided by a simple splined engagement with the hub member with a bias spring urging the armature out of engagement with the inner body.
An arrangement disclosed in application Ser. No. 793,710, filed May 4, 1977 U.S. Pat. No. 4,119,184, issued Oct. 10, 1978 provides these requirements while generating an augmenting engagement force. In this arrangement, circumferentially spaced leaf springs are provided having opposite ends connected respectively to the hub and the armature. This patent application discloses a three leaf spring arrangement. It is difficult to increase the torque capacity of the unit by merely up-sizing the leaf spring elements and the riveted connections utilized to secure the same to the hub and armature, respectively, since the increased sizing would tend to interfere with the pole areas. The springs, being constructed of steel, could potentially cause some leakage of the magnetic flux into the hub member, reducing the electromagnetic flux transmitted by the armature, thus reducing the clutch pull-in force.
Increasing the number of leaf springs, i.e., from three to six in an attempt to provide adequate torque capacity, has led to further difficulties in that the shortened length of the leaf springs produces an increased stiffness of the spring elements, tending to create an excessive resisting force to movement of the armature. Upon wear of the unit such that the air gap existing between the armature and the inner body is increased, the excessively high bias forces exerted by the leaf springs could prevent proper engagement of the clutch resulting in premature failure of the unit.
The thickness of the leaf springs can only be reduced to a limited degree before the torque capacity of the coupling arrangement is compromised.
Accordingly, it is the object of the present invention to provide a coupling arrangement for an electromagnetic clutch of the general type described, i.e., in which a plurality of circumferentially extending leaf spring elements are employed to produce a torsional connection while accommodating limited axial movement and exerting an axial bias force on the connected element, in which a relatively large number of leaf spring elements are incorporated in the coupling arrangement without unduly increasing the stiffness of the leaf spring elements.